Absorption spectra from a mixture of 320 ppm in synthetic air (79% 21% ) were collected in the region from to under conditions in the range of 100–1000 atm and 295–900 K. At 295 K, both bands of the Fermi dyad show the collapse of and branches into a single nearly Lorentzian spectral feature as a result of interbranch line-mixing. At elevated temperatures, the presence of interbranch mixing is also clearly evident as is the presence of several hot bands. The experimental data are modeled using two methods for simulating line-mixed spectra; first, the usual line-by-line approach which relies on the binary impact approximation, and second, a simple band-averaged model proposed by Hartmann and L’Haridon [J. Chem. Phys. 103, 6467 (1995)]. The energy corrected sudden (ECS) approximation is used to generate the relaxation matrix in the first approach. Comparison with the measurement shows that the ECS method does not fit the high density data satisfactorily when adjustable parameters from the literature are used; the level of interbranch mixing must be decreased by about a factor of 2 relative to intrabranch mixing and at least 5% dephasing must be added to the ECS matrix. With these changes, the room temperature data are modeled satisfactorily, but significant discrepancies are still present in the high temperature spectra. On the other hand, the simpler band-averaged model does provide a reasonable estimate of the spectra for all temperatures when best fit values are used for mixing and broadening, but the low density data are not reproduced as well as with the ECS model. Data from high pressure absorption measurements in a 1% NO in mixture as well as a 0.5% in mixture are also presented without analysis, showing the effects of interbranch line-mixing in these spectra.

The and valence excited states have been characterized via resonance enhanced two-photonionization (R2PI) spectroscopy of transitions from the long-lived metastable states of the MgKr van der Waals molecule. Because the excited orbital is quite diffuse and the Kr atom can approach along the nodal axis of the aligned orbital, minimizing repulsion, the state is very strongly bound closely approaching the bond energy of the core ion (for which ). In contrast, the state is more weakly bound although it has a greater bond strength than the lower state (for which is ). We have also observed interesting spin–orbit effects which are attributed to mixing of Kr character into molecular orbitals which are nominally of excited-state character.

The and excited states have been characterized via resonance enhanced two-photonionization (R2PI) spectroscopy of transitions from the long-lived metastable states of the MgXe van der Waals molecule. Because the excited orbital is quite diffuse and the Xe atom can approach along the nodal axis of the aligned orbital, minimizing repulsion, the state is even more strongly bound than the core ion (for which ). The state and the state are much less bound. However, the potential curves of these two states are quite different, and it is suggested that the state is bound only because of substantial mixing of Rydberg character into the wave function. Interesting spin–orbit and spin–spin effects, detected and analyzed from the rotational structure of the vibrational bands, are attributed to mixing of some Xe character into molecular orbitals nominally of excited state character.

Aqueous cluster studies have lead to a reassessment of the electronic properties of bulk water, such as band gap,conduction band edge, and vacuum level. Using results from experimental hydrated electron cluster studies, the location of the conduction band edge relative to the vacuum level (often called the value) in water has been determined to be which is an order of magnitude smaller than most experimental values in the literature. With and making use of the calculated solvation energy of OH in water, the band gap of water is determined to be 6.9 eV. Again, this is smaller than many literature estimates. In the course of this work, it is shown that due to water’s ability to reorganize about charge (1) photoemission thresholds of water or anionic defects in water do not determine the vacuum level, and (2) there is almost no probability of accessing the bottom of the conduction band of water with a vertical/optical process from water’s valence band. The results are presented in an energy diagram for bulk water which shows the utility of exploring the conduction band of water as a function of solventpolarization.

The cyclic cluster has been identified for the first time in Fourier transforminfrared spectra of the products from the laser evaporation of graphite rods trapped in Ar at Measurements on spectra produced using both - and -enriched rods combined with the results of new density functional theory calculations performed in the present work as well as previous calculations by Martin and Taylor, have resulted in the assignment of the most intense infrared active mode, of the cyclic isomer with symmetry. This assignment is based on excellent agreement of the frequency, isotopic shifts, and relative intensities with the theoretical predictions.

Wide angle neutron scattering combined with H/D substitution has been applied to determine the hydrogen bondstructure in glassy and liquid glucose H/D substitution involved only the H atoms belonging to OH groups. The resulting radial distribution function clearly shows the ordering of H atoms, in a way expected from hydrogen bonding. At and 35 °C, at which temperatures glucose is in the glassy state, temperature dependence of the hydrogen bondstructure is clearly observed. The number of hydrogen bonds in the glassy state is similar to that in the crystalline state. At 80 °C, in the undercooled liquid state, the number of hydrogen bonds is 20% lower and the hydrogen bondstructure much less pronounced. The hydrogen bond H–H distance is found to be in the range of 2.4–2.6 Å, similar to the values found in the crystal (2.40 Å) and water at room temperature (2.44 Å). At the lower temperatures, preliminary GROMOS molecular dynamics simulation results of are in reasonable agreement with the experimental data. However, at 80 °C, the simulations seriously overestimate intermolecular structure.

We develop a theory for relating quantum and classical time correlation functions in the context of vibrational energy relaxation. The treatment is based on the assumption that both the quantum and the classical systems are characterized by effective harmonic Hamiltonians with identical normal modes; and the solute-solvent interaction is taken to be linear in the solute vibrational coordinate, but nonlinear in the bath coordinates. We propose an approximate “quantum correction” which allows the determination of the quantum energy relaxation rates from the classical force-force time correlation functions in the limit of large solute’s vibrational frequency. We test the accuracy of this approximate correction against exact numerical results for two forms of the solute-solvent interaction (exponential and power law), and find it to be accurate for a wide range of solute vibrational frequencies and for different solventthermodynamic states. A simple form of the “quantum correction” is proposed for the models based on Lennard-Jones interactions. In all cases it is found that the vibrational relaxation time in a fully quantum system is better approximated by a fully classical theory (classical oscillator in classical bath) than by a mixed quantum-classical theory (quantum oscillator in classical bath).

The photophysical behavior of 3-chloro-7-methoxy-4-methylcoumarin (ClMMC) was studied as a function of the solvent and temperature. The radiative lifetime is essentially solvent independent and its value is totally commensurate with the fluorescence originating from a state as the lowest excited state. From the fluorescence data obtained in 24 solvents plus nine solvent mixtures, and the triplet formation quantum yields for three representative solvents, it was found that the internal conversion rate constant dictates the photophysical behavior of ClMMC and changes of two orders of magnitude occur from nonpolar to polar solvents. From the temperature dependence (20 to ) of the fluorescence lifetimes in five solvents it was found that a change of the internal conversion rate constant of the same order of magnitude occured as above. The rate constants and the activation energies for the radiationless processes were determined. The results show that the reason for the dramatic variation of is the fact that when the state is close lying to there is a decrease of the activation energy of the internal conversion process Increasing solvent polarity increases the energy gap between these states, and decreases the magnitude of the effect. Decreasing the temperature to sufficiently low values, disables the deactivation channel. The interpretation of the foregoing results can be satisfied by either a model involving a fast equilibrium between two close lying and states or in terms of the so-called “proximity effect.”

A theoretical ab initio simulation of the carbon and oxygen KLL and sulfur LMM Auger spectra of carbonyl sulfide is presented and discussed. The underlying vertical double ionization spectrum is computed using a Green’s function method; the Auger intensity distributions are estimated via a two-hole population analysis of the eigenvectors and the main effects of nuclear vibrational motion on the energy position and broadening of the Auger bands are taken into account. The simulation yields accurate spectra, revealing the important role played by the nuclear dynamics effects at the origin of the very different shapes of the three spectra. These effects are particularly striking in the sulfur LMM spectrum, which is additionally characterized by evident spin–orbit coupling in the decaying state.

We report experiments studying the fluorescence and two-photon excitation spectroscopy of xenon dimers and small clusters formed in supersonic jets. Under thermodynamic conditions for maximum dimer density, determined by two-photon resonant multiphoton ionized time-of-flight (TOF) spectroscopy, no fluorescence of free bound excimers correlating to Xe* or was observed; very weak excimer fluorescence was observed for the excimer correlated with Xe+Xe* Comparing the observed atomic fluorescence and measurements of the monomer–dimer ratio, we estimate predissociation lifetimes on the order of of the expected radiative lifetime. TOF spectra are consistent with predissociation for all excimers except those correlating to At higher nozzle stagnation pressures, we observed fluorescence from vibrationally or electronically relaxed excimers imbedded in helium clusters with 〈〉 most probably 6 and 13. We report dispersed spectra, and using modeled reflection spectra, we assign the fluorescence spectra to and

The three pulse photon echo peak shift technique was used to study solvation dynamics in acetonitrile (297 K), methanol (297 and 323 K), and ethylene glycol (297 and 397 K) utilizing the tricarbocyanine laser dye, IR144, as a probe. The spectral density, ρ(ω), governing the solute-solvent interaction was obtained for each solvent and temperature through numerical fitting of the three pulse photon echo peak shift relaxation using finite temporal-duration optical fields. An ultrafast three pulse photon echo peak shift relaxation, ascribed to the inertial component, was nearly identical for ethylene glycol at 297 and 397 K; this indicates the spectral density is essentially temperature independent from 10 to over this temperature range. Conversely, the low-frequency spectral density obtained from three pulse photon echo peak shift relaxation of ethylene glycol at 297 and 397 K showed a strong temperature dependence which cannot be predicted using harmonic bath models. We calculated spectral densities for ethylene glycol, acetonitrile, and methanol using the simple dielectric continuum model and the dynamical mean spherical approximation, using where possible, the relative permittivity constants calculated from experimental far-infrared absorption data and dielectricdispersion data. Additionally, we calculated spectral densities in terms of the extended reference interaction site model for methanol and acetonitrile. These calculated spectral densities describe our experimental methanol and acetonitrile photon echo better than all other solvation model spectral densities. Our results give insight into the domain of applicability of the harmonic model of liquid dynamics.

The pure rotational spectrum of in its ground state was recorded in the 198–384 GHz spectral region using a dc-sputtering absorption spectrometer. The determined hyperfine parameters are and where a three standard deviation error estimate is given in parentheses. A plausible molecular orbital description for the state based upon the determined parameters is given.

A total of 3261 ab initio energies calculated at the RHF/MP2 level were used to obtain an analytical representation of the potential energy surface (PES) for the title reaction considering all the vibrational degrees of freedom. The analytical potential is constructed by switching three local representations of the PES utilizing a distancelike function, and it reproduces well the ab initio energies up to 20 kcal/mol above the dissociation threshold with the root-mean-square (rms) deviation equal to 1.5 kcal/mol. Two types of classical trajectory studies, i.e., power spectra calculations and product-state distribution analysis, were performed to assess the reliability of the present potential function. The results were found to be in good agreement with the available experimental ones.

Quantum state distributions for nascent OH and OD fragments generated by Franck–Condon “forbidden” 193 nm photodissociation of and are reported, with the two isotopomers initially prepared in their zero-point vibrational and lowest ortho/para nuclear spin allowed rotational states (i.e., and in a 3:1 ratio for and 1:2 ratio for ) by cooling in a slit supersonic expansion. Product state distributions are probed via OH/OD laser-induced fluorescence(LIF) with cylindrical mirror collection optics optimized for the slit expansion geometry, which makes photodissociation studies feasible with cross sections as low as The OH and OD fragments are formed exclusively in but with highly structured quantum state distributions in rotational, Λ-doublet, and fine structure levels ( and ) that exhibit qualitatively different trends than observed in previous jet photolysis studies at 157 nm in the Franck–Condon “allowed” regime. The relative OH/OD fragment yields at 193 nm indicate a times greater propensity for OH vs OD bond cleavage in than which is more than three-fold smaller than predicted from full three-dimensional quantum scattering calculations on ground and first excited statepotential surfaces. One-dimensional semiclassical calculations of the Franck–Condon overlap matrix elements confirm these discrepancies to be considerably outside uncertainties associated with the ground and excited statepotential surfaces. These results indicate that the photodissociationdynamics for this benchmark system are not yet fully understood and suggest either non-Born–Oppenheimer effects or contributions from other electronic surfaces may be important in the extreme non-Franck–Condon photolysis regime.

A detailed quasiclassical trajectory study of the reaction is performed based on ab initiopotential-energysurfaces of the and states. The study is aimed at generating a database of thermally averaged and state-specific rate constants needed for accurate simulations of NO kinetics in high-temperature flow processes. The rate constants obtained show good agreement with the available experimental data and with other quasiclassical trajectory calculations. It is found that the reactant internal energy of the reaction is less effective in enhancing the rate than in thereaction. An analysis of the product vibrational energy shows that NO formed by thereaction has a non-Boltzmann distribution. It is also found that the most populated NO vibrational level is determined by the reactant vibrational energy, while the terminal slope of the NO vibrational distribution is a strong function of the reactant translational temperature.

The reactive collisions of protons with methane molecules at 30 eV in the laboratory frame are studied with the electron nuclear dynamics (END). The results from this theoretical approach, which does not invoke the Born–Oppenheimer approximation and does not impose any constraints on the nuclear dynamics, are compared to the results from time-of-flightmeasurements. Total differential cross sections and integral cross sections as well as fragmentation ratios and energy loss spectra are discussed.

The dissociation of complexes was investigated using mass analyzed threshold ionization spectroscopy. All ion state spectra of the cationic complexes exhibit low-frequency vibrational progressions of van der Waals bending modes, which indicate a significant structural change of the complexes upon ionization. Upper limits for the dissociation thresholds in the cationic state could be determined for all complexes. In the case of and two fragmentation thresholds could be observed.

A time dependent theory for radiative recombination induced by strong pulses is presented. Analytic solutions in the adiabatic limit are derived and found to be in excellent agreement with exact numerical solutions. Both the pump-before-dump “intuitive” and dump-before-pump “counter-intuitive” schemes are considered. Resonantly enhanced two-photon recombination of ultracold atoms is shown to be an efficient mechanism for the production of ultracold molecules. We have performed detailed calculations on the radiative recombination of cold Na atoms by short laser pulses. Our calculations show that, per pulse, it is possible for up to 97% of all head-on Na-Na colliding pairs to end up as , translationally cold Na molecules. We show that these findings, translated to thermally cooled ensemble conditions, mean that the fraction of Na atoms at Kelvin which can be recombined by a pulse of 20 ns duration and peak intensity, to form molecules is per pulse. With the above parameters, a laser operating at 100 Hz can convert half of an ensemble of cold atoms to cold molecules in 25 min. The efficiency of the process can be increased by going to longer pulses of lower intensity, by going to lower temperatures or by increasing the density of the ensemble. In particular, the “counter-intuitive” scheme which allows for use of longer pulses of lower intensities, with no spontaneous emission losses, considerably increases the yield.

We have carried out a systematic crossed molecular beam study of the hydrogen exchange reaction in the H+D HD+D isotopic form at two collision energies: 0.53 and 1.28 eV. The Rydberg atomtime-of-flight method was used to measure the D-atom product angle-velocity distribution. For the first time ro-vibrational quantum state resolved differential cross sections for the title reaction were measured, which can directly be compared to theoretical predictions at this detailed level. Experimental results are compared to theoretical predictions from both quasi classical and quantum mechanical calculations on different potential energy surfaces as well as to earlier experiments. A general good agreement is found for the converged quantum mechanical calculations with indications that the Boothroyd-Keogh-Martin-Peterson potential energy surface is better suited to describe the dynamics of the reaction. For the higher collision energy the quasi classical trajectory calculations reproduce the experimental data quite well, whereas they fail to describe the situation at the lower collision energy especially with respect to angular resolved differential cross sections.

Infrared cavity ringdown laser absorption spectroscopy (IR-CRLAS) is employed to determine absolute methyl radical concentrations in a 37.5 Torr laminar methane/air flame. IR-CRLAS rovibrational absorption spectra of the fundamental band system near are combined with temperature measurements to obtain methyl radical concentrations as a function of height above the burner surface. These data are compared with flame chemistry simulations under both stoichiometric and rich flame conditions. Issues regarding the applicability of IR-CRLAS for combustion studies are discussed, including the uncertainties present for the specific case of methyl radical. These IR-CRLAS measurements indicate the ability to monitor reactants, intermediates, and products within a narrow spectral window, and, to our knowledge, constitute the first infrared detection of a polyatomic radical in a flame.